1. Field of Invention
The present invention is directed to turbine systems for the generation of electrical power and/or the production of fresh water from ocean tidal and/or current (gyre) flows, and more specifically, to a floating, yawing spar tidal turbine.
2. Description of Related Art
Ocean currents are a major, largely untapped energy resource. The potential for ocean current electric power generation in the United States is as much as 185 TWh/yr., comprising both tidal and gyre currents, which have the advantage of delivering clean, renewable, predictable power to the coastal transmission grid, which are usually in close proximity to high load centers. Research and development in this area is driven by the need to generate electricity from renewable energy resources, particularly in view of the rising level of CO2 and methane in the earth's atmosphere from the combustion of carbon fuels and the resulting disruptive impact on climate from global warming.
An ocean current is a continuous, directed movement of seawater generated by the forces acting upon this flow, such as breaking waves, wind, Coriolis effect, evaporation, temperature, salinity differences, and tides caused by the gravitational pull of the Moon and the Sun. Depth contours, shoreline configurations and interaction with other currents influence a current's direction and strength. Ocean gyre currents are relatively constant and near our coastlines, generally flow in one direction, in contrast to periodic tidal currents that reverse in flow direction due to gravitational forces. Harnessing a slow moving fluid to generate power has been effectively accomplished with wind turbines. While ocean currents move slowly relative to typical wind speeds, they carry a great deal more energy because of the density of water, which is more than 800 times that of air. The following table illustrates the average electrical power density as a function of wind or current flow speeds for a wind turbine or marine turbine, respectively.
Wind TurbineMarine TurbineAverage WindAverageSpeed (typicalAverage PowerFlow SpeedAverage Powerrange - m/s)Density W/m2(typical range - m/s)Density W/m26.01320.994987.52581.208868.53761.43150010.06131.602101
With gyre currents, the constancy of flow also provides the opportunity for steady electric power delivery, compared to the intermittency of wind and solar. Because of these physical properties, ocean currents contain an enormous amount of energy that can be captured and converted to a usable form.
The United States, United Kingdom, Japan, and other countries are pursuing ocean current energy. Wind turbine technology is mature and generally adaptable to marine conditions, therefore the principal challenge to economically harnessing currents has been with the turbine platform topology and its station holding approach. For wide commercial deployment, turbines must be easy to transport, install, have accessibility to the turbine power deck for operations and maintenance, and generate power with levelized cost of energy (LCOE) comparable to wind turbines and photovoltaic systems.
For ocean current energy to be utilized successfully on a commercial scale, a number of engineering and technical challenges need to be addressed, including: avoidance of blade cavitation (bubble formation); prevention of marine growth buildup; reliability (since at-sea maintenance costs are potentially high); efficient methods of deployment; corrosion resistance; and anchoring and mooring methods. System reliability is of particular importance, since the logistics of at-sea maintenance is limited by accessibility, in windows of acceptable weather and sea-states, adding to the costs of maintenance services. Any system deployed in the ocean must be able to survive extreme waves and storms, which raises the capital cost and maintenance, and have minimal impact on the marine environment, such as fishing grounds, beach shoreline, and be compatible with ocean navigation.
Korean Patent No. 936907 to Kim discloses an ocean floor mounted, two rotor tidal electrical power generating system in which a main body automatically rotates so that a rotor always faces the tidal flow. This system is expensive due to the structural requirements necessary to resist the overturning moment of the whole structure due to the thrust load of the current on the rotors. This limits deployment to shallow locations. Installation is costly, since it is only possible to perform during short periods of time between tidal flows. This permanent type of installation makes it challenging to return the structure to the shore base facility for long-term servicing. Servicing just one rotor results in raising all the rotors above the surface and shutting down both rotors (not just the one requiring servicing) of the system resulting in significant loss of power production. Moreover, both rotors, even when in prime workable condition, must operate simultaneously since a shutdown of one rotor would turn the rotor support structure toward oblique alignment with the flow rather than squarely facing the flow, significantly reducing power production.
U.S. Pat. No. 7,307,356 to Fraenkel discloses a dual rotor marine current turbine mounted on the ocean floor. This system is also expensive due to structural requirements to deal with the overturning of the whole structure resulting from thrust load of the current on the rotors. Also, installation and securing to the ocean floor is only possible during short periods of time between tidal flows. Rotors and support structure can be raised for servicing, however all power generation is shut down if only one rotor requires servicing. Rotors and their support structure do not yaw. Rather, the blades reverse direction for change in tidal flow direction. This means that the rotors are not squarely facing the flow when the flow in one direction is a little different than the flow in the opposing direction resulting in reduced power production.
Great Britain Patent No. 2,447,774 to Fraenkel discloses a deep water current dual turbine system anchored to an ocean floor. If one rotor malfunctions, requiring servicing, the entire system must be shut down and brought to the surface. In a tidal flow, this would be difficult, as the flow in one direction drops off and waters calm, only a brief window in time is available for servicing operations before the flow reverses, at which point the whole anchored structure must swing around to an opposite position on the surface due to the opposite flow direction. There is no surface accessibility to the turbine drivetrains for servicing. This design appears costly, complex, and problematic to service. This design is based on the rotors downstream of the spar and yawing of the rotors to face the flow downstream of the spar. When the tide flow changes yawing is delayed until the flow is sufficiently strong, to drag the non-operating rotors around to a downstream position. The loss of one turbine operating will cause the balance of the turbines to yaw away from squarely facing the flow, significantly reducing the power generated by the whole system. This results in lost production and poor equipment utilization.
In summary, known prior art systems are not capable of producing cost-effective, utility-scale electrical power output to meet modern energy needs. What is needed is a system for efficiently capturing power from ocean or tidal currents, to generate electric power or produce desalinated water, which is cost effective to manufacture, deploy, and maintain.